Fretting fatigue testing device and method suitable for seabed connector

Abstract

The invention relates to a fretting fatigue testing device and method suitable for a seabed connector, and the device comprises a frame which is of a square frame structure; the number of the clamps is two, the first ends of the two clamps are connected with the inner wall of the frame through cushion blocks, and the second ends of the two clamps are used for clamping the two axial ends of a sample; the number of the loading rods is two, the two loading rods are vertically arranged at the two axial ends of the sample, and the ends, close to the sample, of the two loading rods are connected with the two cushion blocks through connecting rods; the number of the actuators is four, the actuators are arranged on the four outer walls of the frame, and the four actuators are connected with the two loading rods and the two clamps respectively and used for applying loads to the sample. The device comprehensively considers the influence of the normal load, the shear load, the overall tension and the torque on the fretting fatigue life.

Description

Technical Field

[0001] This invention relates to a fretting fatigue testing device and method suitable for submarine connectors, belonging to the field of fretting fatigue testing technology. Background Technology

[0002] Subsea wellhead connectors are critical components in subsea oil and gas extraction. They are connected to the wellhead via a locking device. Under normal operating conditions, these connectors must withstand internal pressure and cyclic bending loads caused by waves, while preventing wellhead separation.

[0003] Fretting fatigue is an additional phenomenon besides traditional high-cycle fatigue (crack initiation from the wellhead profile groove) that needs to be considered in the design documents. Fretting fatigue occurs between contact bodies undergoing cyclic loading. Due to high local stress and surface damage, fatigue cracks initiate at the contact edges and propagate when sufficiently high volumetric stresses are present. If both local and volumetric stress ranges are sufficiently high, fretting fatigue can become a potential failure mode for the component. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a fretting fatigue testing device and method suitable for submarine connectors. This device comprehensively considers the influence of normal load, shear load, overall tension, and torque on fretting fatigue life.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A fretting fatigue testing device suitable for submarine connectors, comprising: The frame is a square frame structure; The clamps are in the form of two clamps. The first end of each clamp is connected to the inner wall of the frame through a pad, and the second end of each clamp is used to hold the two ends of the sample along the axial direction. There are two loading rods, which are vertically arranged at both ends of the sample. The ends of the two loading rods closest to the sample are connected to the two pads through connecting rods. The actuators, numbered four, are disposed on the four outer walls of the frame. The four actuators are respectively connected to the two loading rods and the two clamps, and are used to apply loads to the sample.

[0006] Preferably, in the fretting fatigue testing device for submarine connectors, a first gasket and a second gasket are provided on the wall surface where the sample contacts the two loading rods.

[0007] A second aspect of the present invention provides an experimental method for a fretting fatigue testing device suitable for submarine connectors, comprising the following steps: The sample is held in place using clamps; An axial load is applied to the sample by an actuator to obtain the normal load P, and the torque M is calculated. A load perpendicular to the axial direction of the specimen is applied to the specimen using an actuator to obtain the shear load Q and volumetric tension. ; By changing the normal load P, shear load Q, and volume tension The study investigated the relationship between four parameters and fretting fatigue life.

[0008] The experimental method for the fretting fatigue testing device suitable for submarine connectors, preferably, uses the following formula for calculating the torque M:

[0009]

[0010] In the formula, P For normal load; - a arrive b This refers to the contact area.

[0011] The experimental method for the fretting fatigue testing device suitable for submarine connectors, preferably, uses a formula for calculating the torque M based on the following assumptions: The frictional contact between the sample and the first and second gaskets is idealized as an inclined, flat, circular punch subjected to a normal load P, a torque M, a shear load Q, and overall tension. Its function is to have a contact range spanning [-a, c], with a flat portion 2b and corresponding radii R1 and R2 at the ends.

[0012] The experimental method for the fretting fatigue testing device suitable for submarine connectors, preferably, involves changing the normal load P, shear load Q, and volumetric tension. This study investigates the relationship between four parameters and fretting fatigue life. N The relationship formula is as follows:

[0013]

[0014]

[0015] In the formula, This represents the change in the tangent multiplier. This represents the change in the scaling factor; d The size of the sliding area; The coefficient of friction; This is the scaling factor; N This refers to the fretting fatigue life.

[0016] The experimental method for the fretting fatigue testing device suitable for submarine connectors, preferably, involves changing the normal load P, shear load Q, and volumetric tension. The study investigated the relationship between four parameters and fretting fatigue life. The specific experimental procedure is as follows: By continuously changing the amplitude of the shear load Q while keeping other parameters constant, the relationship between it and fretting fatigue life was studied. Continuously changing volume tension The maximum value of R is set to 0.1, while other parameters remain unchanged. The relationship between R and fretting fatigue life is studied. With a fixed shear load Q, four different values ​​of normal load P are set to study the relationship between the average value and amplitude of normal load P and fretting fatigue life. The shear load Q is set to another fixed value, and the same four normal load P values ​​are set to study the relationship between the average value and amplitude of the normal load P and the fretting fatigue life. The shear load Q is set to a certain value, the average value of the normal load P is set to a certain value, and the amplitude is set to 0. The relationship between the average value of the normal load P and the fretting fatigue life is studied. The shear load Q is set to a fixed value, the average value of the normal load P is set to a fixed value, and the amplitude is changed to study the relationship between the amplitude of P and the fretting fatigue life.

[0017] The experimental method for the fretting fatigue testing device suitable for submarine connectors, preferably, involves setting a certain value for the shear load Q and setting the conditions for the values ​​of four normal loads P as follows: High average value, no amplitude; low average value, 15% amplitude; high average value, 15% amplitude; low average value, no amplitude.

[0018] The present invention has the following advantages due to the adoption of the above technical solutions: 1. The fretting fatigue testing apparatus and method for submarine connectors of the present invention do not match individual components such as pressure and slip displacement, but rather maintain external characteristic solutions. These solutions revolve around the process regions of slip and fatigue crack initiation. One solution is used for normal load, and the other for shear, scaling the local stress state, thereby simplifying the process of fretting fatigue life measurement.

[0019] 2. The device of the present invention comprehensively considers the influence of normal load, shear load, overall tension and torque on fretting fatigue life. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a fretting fatigue testing device provided in an embodiment of the present invention; Figure 2 This is an experimental flowchart of the fretting fatigue testing device provided in this embodiment of the present invention; Figure 3 This is a schematic diagram of an embodiment of the present invention, which idealizes the frictional contact between the sample and the first and second gaskets as an inclined, flat, circular punch. Figure 4 for Figure 3 Traction force diagram along the contact interface during the middle part of the slippage; Figure 5 for Figure 3 Singular asymptotes of the bounded square root and tangential square root in the direction of normality; Figure 6 The main body material of the underwater wellhead connector is F22, and this is a physical image of the material used in the fretting fatigue life test. The attached figures are labeled as follows: 1-Clamp; 2-Actuator; 3-Frame; 4-Bracket; 5-Padded block; 6-Loading rod; 7-Sample. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0022] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," "third," "fourth," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect.

[0023] For ease of description, spatial relative terms may be used in the text to describe the relationship of one element or feature relative to another element or feature as shown in the figure. These relative terms include, for example, "inside," "outside," "middle," "outer," "below," "above," etc. Such spatial relative terms are intended to include different orientations of the device in use or operation, other than those depicted in the figure.

[0024] One existing technology discloses an in-situ fretting fatigue testing device and method. The device includes an axial fatigue loading system and a fretting loading system, wherein the axial fatigue loading system works in conjunction with the axial loading system to achieve fretting fatigue of the specimen. The in-situ fretting fatigue testing device, placed on a sample rotation stage with a light source, can be used in conjunction with a synchrotron radiation source for imaging and diffraction characterization of the specimen. It can characterize the evolution of damage defects such as cracks and residual stress during fretting fatigue loading in real time, in-situ, and visually, clearly and accurately reflecting the damage evolution characteristics inside the material under fretting conditions. This provides indispensable high-end technical equipment for studying crack initiation and propagation behavior during the fretting fatigue failure process of materials. However, the use of the synchrotron radiation source in this invention involves high costs and complex operation, and synchrotron radiation sources may raise radiation safety concerns, requiring additional safety measures.

[0025] Prior art discloses a device for fretting fatigue testing, including an axial load loading device, a normal load loading device, and two fretting pads. The axial load loading device can apply an axial load to the fretting specimen. The normal load loading device includes a displacement driving device, two normal load loading ends, and a synchronous displacement transmission device. The two normal load loading ends are respectively disposed on opposite sides of the fretting specimen, and the two normal load loading ends can move closer to and further away from the fretting specimen. The displacement driving device drives one of the normal load loading ends to move and can drive the other normal load loading end to move closer to or further away from the fretting specimen synchronously through the synchronous displacement transmission device. The two fretting pads are respectively installed on the side of the two normal load loading ends facing the fretting specimen. The device provided by this invention can ensure good alignment and equal contact pressure of the fretting pads on both sides during loading, and facilitates switching between the types of fretting pads. However, this device mainly focuses on convenient loading and good alignment during loading.

[0026] Existing technology three discloses a method for monitoring fretting fatigue state. First, an acoustic emission device is selected based on the fretting fatigue characteristics. Then, an acoustic emission sensor is installed on the test piece. Next, relevant parameters of the acoustic emission device are adjusted to accurately monitor changes in the acoustic emission signal during the fretting fatigue test. Finally, the fretting fatigue state and the initiation time of fretting fatigue cracks are analyzed through changes in the acoustic emission signal. This invention can study the fretting fatigue state under different materials and connection structures and overcomes the difficulty in determining the initiation life of fretting fatigue cracks, thus having significant practical engineering value. However, the acoustic emission device is costly, and its effectiveness is affected by material properties, which can influence the judgment of actual fretting fatigue life. Furthermore, it lacks universality. In contrast, the device design of this invention prioritizes cost-effectiveness and achieves universality by simplifying parameters.

[0027] Based on the above-mentioned technical problems, the present invention provides a fretting fatigue testing device and method for simulating complex load conditions in the deep sea environment. This device abandons the traditional method of comprehensively considering the influence of normal load, shear load, overall tension and torque on fretting fatigue life.

[0028] like Figure 1 As shown, the fretting fatigue testing device for submarine connectors provided by this invention includes the following components: a frame 3, which is a square frame structure; two clamps 1, the first ends of which are connected to the inner wall of the frame 3 via pads 5, and the second ends of which are used to clamp the two ends of the specimen 7 along the axial direction; two loading rods 6, which are vertically arranged at the two ends of the specimen 7 along the axial direction, and the ends of which are close to the specimen 7 are connected to the two pads 5 via connecting rods; and four actuators 2, which are arranged on the four outer walls of the frame 3, and the four actuators 2 are respectively connected to the two loading rods 6 and the two clamps 1, for applying loads to the specimen 7. A first pad and a second pad are provided on the wall surface of the specimen 7 that contacts the two loading rods 6.

[0029] A second aspect of the present invention provides an experimental method for a fretting fatigue testing device suitable for submarine connectors, comprising the following steps: Step 1: Hold the sample 7 in place using clamp 1; Step 2: Apply an axial load to the specimen 7 using actuator 2 to obtain the normal load P, and calculate the torque M; Step 3: Apply a load perpendicular to the axial direction of specimen 7 to specimen 7 using actuator 2 to obtain the shear load Q and volumetric tension. ; Step 4: By changing the normal load P, shear load Q, and volumetric tension The study investigated the relationship between four parameters and fretting fatigue life.

[0030] Specifically, in step 2, as follows Figure 3 As shown, the frictional contact between sample 7 and the first and second gaskets is idealized as an inclined, flat, circular punch subjected to a normal load P, a torque M, a shear load Q, and overall tension. The contact range spans [-a, c], with a flat portion 2b and corresponding radii R1 and R2 at the ends. This type of contact is called incomplete contact. This does not mean that the contact range increases with the applied normal load P; unlike complete contact, where the contact edges are sharp (i.e., not smooth), the contact range is independent of the applied normal load. Nucleation lifetime and potential cyclic stress leading to crack propagation. Assuming the friction coefficient f is finite, the contact can be in one of three states: 1. If the shear force is large enough to cause complete sliding, Coulomb's law will be in a limiting state along the entire contact interface. Therefore, .

[0031] 2: When the shear force is insufficient to cause complete sliding, the contact may become completely stuck, meaning there is no relative movement between any opposing surface particles.

[0032] 3: Partial sliding; micro-motion damage may accumulate at the site of sliding, such as... Figure 4 As shown.

[0033]

[0034]

[0035]

[0036]

[0037] In the formula, For normal load, For torque, For shear load, - a arrive b For the contact area, This is the length of the sliding zone.

[0038] The edges of any indenter will be rounded because perfectly sharp edges do not exist in reality; pressure distribution at the contact edge is always bounded by the square root. This univariate approximation can be used based on scaling factors. Define the traction force of the near-edge normal. p （s） ,as follows:

[0039] Where s is measured inwards from the contact edge, and d is the size of the sliding zone. Using the finite element method, the multiplier... The value can be used as the near-edge pressure gradient, and will... The ratio is plotted as S→0.

[0040] Assuming the coefficient of friction is initially high enough to prevent all slippage, if the contact half-width is 'a', the tangential traction caused by the shear force Q will be singular at the edges.

[0041]

[0042] If the tangential load is replaced by the long-range overall tension σ, a shear traction force will be generated:

[0043] If both shear force Q and long-range tension σ exist simultaneously, the superimposed shear forces will be subtracted when x < 0 and added when x > 0, by shifting the coordinates to the left edge (x = a) Translation: Let s = a + x. Its asymptotic form can be easily found. By expanding the shear force in the vicinity of this point (s→0) using a series, and neglecting higher-order terms, we can write it as:

[0044] Among them, the tangent multiplier for:

[0045] Once the local contact loads Q and σ are determined through finite element analysis, the following can be calculated: The value, or, if the corresponding "no-slip" condition is set in the finite element program, can be obtained by drawing... Determined by the product as s→0 , Figure 5 This diagram illustrates a non-holonomic contact. The square-root bounded and square-root singular asymptotes represent the normal and tangential traction forces near the contact edge, respectively.

[0046] In the sliding region, the magnitude of the contact force is determined by a scaling factor. This indicates that if a coefficient of friction is specified... The magnitude of the shear force is also determined by the scaling factor. This means that all that remains to be determined is the range d of the sliding area.

[0047] As can be seen from the above, in any cyclic loading problem, the size of the slip zone is independent of the average value of Q or σ, while and As an important parameter, it affects fretting fatigue life. N .

[0048]

[0049]

[0050]

[0051] In the formula, This is the change value of the tangent multiplier (obtained mathematically and without a precise meaning); d The size of the sliding area; The coefficient of friction; Let P be the scaling factor. It can be seen that P is the normal load, Q is the shear load, and Q is the volumetric tension. The torque M will affect the fretting fatigue life. N It has an impact.

[0052] like Figure 1 As shown, the experimental setup requires loads with four degrees of freedom (P normal load, Q shear load, and volumetric tension). And torque M) to match any contact, but the device used here only allows 3 (P, Q and) The fourth parameter "M" corresponds to the rotation of the shims (redistributing the normal load so that one edge bears more load than the others). The load values ​​to be applied are entered into the control panel of the control system. The dedicated actuators 2 responsible for each load transfer the load to the specimen 7. The normal load "P" is controlled by the dedicated actuator 2 at the bottom of the device. This actuator 2 applies the load through the loading rod 6, the first shim, the specimen, and the second shim, and then the load is reflected through the test frame 3. The two actuators 2 on both sides of the device apply shear loads and volumetric tension. The average value of the tensions applied by these two actuators 2 is the volumetric tension. The difference between them is the shear “Q” applied to the contact, which is reflected through the contact interface and through a set of rods to the test frame 3.

[0053] In these tests, the effects of four load parameters on fretting fatigue life were investigated. These parameters are: the average value of "P", the amplitude of "P", and the amplitude of "Q" (the shear is completely reversible, so the average value is always zero). "Maximum value (R ratio maintained at 0.1). The test was conducted at a loading frequency of 10Hz, with a run ending at 2,000,000 cycles. Details are as follows:" Experiment 1: Effect of shear load on fretting fatigue life: The amplitude of "Q" was continuously changed while other parameters remained constant. The contact life showed a roughly linear change over a wide range of shear loads.

[0054] Experiment 2: The effect of volumetric tension on fretting fatigue life: continuously changing " "Maximum value, R is set to 0.1, other parameters remain unchanged."

[0055] Experiment 3: The effect of average and amplitude of normal load on fretting fatigue life: 3.1: The tests were conducted under the same shear amplitude of 720 N / mm, and were performed under four conditions of "P": Condition 1: High average value (6000 N / mm), no amplitude; Condition 2: Low average value (3500 N / mm), 15% amplitude; Condition 3: High average value (6000 N / mm), 15% amplitude; Condition 4: Low average value (3500 N / mm), no amplitude.

[0056] Under these conditions, the amplitude of "P" has little impact on lifespan, and the average value of "P" is positively correlated with lifespan.

[0057] 3.2: The shear amplitude was reduced to 350 N / mm (to prolong the specimen life so that the effect of normal load changes can be seen more clearly).

[0058] Under these conditions, the conclusions are similar to those in 3.1, further confirming that the amplitude of “P” has little impact on lifespan, and that the average value of “P” is positively correlated with lifespan.

[0059] 3.3: The shear amplitude is set to 350 N / mm, the average value of P is set to 4000 N / mm, and the amplitude of P is set to 0 N / mm.

[0060] Specimens with a P value higher than 6145 N / mm face the risk of severe yielding, while specimens with a P value lower than 1902 face the risk of complete contact slippage. Therefore, an intermediate value of 4000 N / mm was chosen to verify how the contact life varies between the average P values ​​at the two extremes.

[0061] 3.3: The test results are consistent with the trend identified in 3.2, that is, the lifespan changes roughly linearly and positively with the average value of P.

[0062] 3.4: The shear amplitude remains at 350 N / mm, the average value of P is 4000 N / mm, and the amplitude of P is changed. Experimental results show a weak negative linear correlation between lifetime and the P amplitude. The maximum variation in lifetime during this phase of testing was approximately 200,000 cycles, which is relatively small compared to the dispersion observed in tests conducted under the same conditions. Furthermore, this cyclic variation may cause the specimen to move by an amount similar to the change in contact dimensions, thus "wiping away" any cumulative surface damage. These results indicate that normal cyclic loads can be neglected in connector design, and that the average value of "P" is strongly positively correlated with fatigue life.

[0063] The fretting fatigue testing method for subsea connectors disclosed in this invention does not focus on individual components such as matching pressure and slip displacement, but rather maintains external characteristic solutions that revolve around the slip and fatigue crack initiation process regions. These solutions, one for normal load and one for shear, scale the local stress state, thereby simplifying the fretting fatigue life measurement process.

[0064] The following section combines specific experimental tests to... and Fretting fatigue life N The relationship will be explained in detail.

[0065] The main body material of the underwater wellhead connector is F22, such as... Figure 6 As shown, 30 samples were selected for this experiment. All fretting fatigue tests were conducted at a frequency of 10 Hz and a friction coefficient of [missing value]. The metal-to-metal configurations utilize different incomplete contact geometries and different load ranges.

[0066] The fatigue test results of the statistical specimens are shown in Table 1. and The value is obtained by measuring DIC (a known method for full-field image analysis based on grayscale digital images).

[0067] Table 1

[0068] For each experimental data point (i.e., an experimental condition that leads to a specific lifetime), the corresponding value is calculated using the asymptotic method described in the theoretical derivation process. and The value of .

[0069]

[0070]

[0071] according to The value divides all data points into three groups: low, medium, and high. - Observe the changing trends of these three sets of data in the table.

[0072]

[0073] As can be seen from Table 2-4, in each Within each group, the data points are clearly displayed. The larger the size, the shorter the fatigue life. At the same time, when comparing different groups, under the same conditions... Down, The higher the group, the shorter its fatigue life.

[0074] Meanwhile, power-law regression was used to fit the experimental data to obtain the expression for the fretting fatigue life N:

[0075] The above formula shows and It has a significant impact on fatigue life, showing a negative correlation.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fretting fatigue testing device suitable for submarine connectors, characterized in that, The utility model relates to a test device for micro-motion fatigue test of sample, which comprises: a frame (3) which is a square frame structure; two clamps (1), the first end of the two clamps (1) is connected with the inner wall of the frame (3) through a cushion block (5), and the second end of the two clamps (1) is used for clamping the two ends of the sample (7) in the axial direction; two loading rods (6) which are vertically arranged at the two ends of the sample (7) in the axial direction, and the end of the two loading rods (6) close to the sample (7) is connected with the two cushion blocks (5) through a connecting rod; four actuators (2) which are arranged on the four outer walls of the frame (3), and the four actuators (2) are connected with the two loading rods (6) and the two clamps (1) respectively and are used for applying load to the sample (7).

2. The fretting fatigue testing device suitable for a subsea connector according to claim 1, characterized in that, The wall surface of the sample (7) in contact with the two loading rods (6) is provided with a first gasket and a second gasket.

3. An experimental method for a fretting fatigue testing device suitable for a subsea connector, characterized in that, The utility model relates to a test device for micro-motion fatigue test of sample, which comprises: clamping the sample (7) by the clamp (1); applying axial load to the sample (7) by the actuator (2) to obtain normal load P and calculate the moment M; The load is applied to the sample (7) by the actuator (2) in a direction perpendicular to the axial direction of the sample (7) to obtain a shear load Q and a volume tension ; By changing the normal load P, shear load Q and volume tension , the relationship between the four parameters and the fretting fatigue life is studied.

4. The experimental method for a micro- fatigue testing device suitable for subsea connectors according to claim 3, characterized in that, the calculation formula of the moment M is as follows: In the formula, P F is the normal load; a to b is the contact area.

5. The experimental method for a micro- fatigue testing device suitable for subsea connectors according to claim 4, characterized in that, the calculation formula of the moment M is based on the following assumptions: The rubbing contact of the test specimen (7) with the first and second gaskets is idealized as an oblique flat punch, subjected to a normal load P, a moment M, a shear load Q and an overall tension , with a contact range spanning [-a, c], having a flat portion 2b and radii R1 and R2 of the respective ends.

6. The experimental method for a micro- fatigue testing device suitable for subsea connectors according to claim 5, characterized in that, The relationship between the four parameters and the fretting fatigue life was studied by changing the normal load P, the shear load Q and the volume tension The relationship between the four parameters and the fretting fatigue life was studied by changing the normal load P, the shear load Q and the volume tension N The relationship between the four parameters and the fretting fatigue life was studied by changing the normal load P, the shear load Q and the volume tension wherein is the change in tangent multiplier; is the change in scaling factor; d is the size of the sliding zone; is the coefficient of friction; is the scaling factor; N is the fretting fatigue life.

7. The experimental method for a micro- fatigue testing device suitable for subsea connectors according to claim 6, characterized in that, By changing the normal load P, shear load Q and volume tension , the relationship between the four parameters and the fretting fatigue life was studied. The specific experimental process is as follows: continuously changing the shear load Q amplitude, keeping other parameters unchanged, and studying the relationship between the shear load Q and the micro-motion fatigue life; Constantly changing volume tension Maximum, R takes 0.1, other parameters remain unchanged, to study its relationship with the fretting fatigue life; setting four normal load P value conditions when the shear load Q takes a certain value, and studying the relationship between the average value and amplitude of the normal load P and the micro-motion fatigue life; setting the same four normal load P value conditions when the shear load Q takes another certain value, and studying the relationship between the average value and amplitude of the normal load P and the micro-motion fatigue life; setting the same four normal load P value conditions when the shear load Q takes another certain value, and studying the relationship between the average value and amplitude of the normal load P and the micro-motion fatigue life; setting the same four normal load P value conditions when the shear load Q takes another certain value, and studying the relationship between the average value and amplitude of the normal load P and the micro-motion fatigue life.

8. The experimental method for a micro- fatigue testing device suitable for subsea connectors according to claim 7, characterized in that, the four normal load P value conditions are as follows when the shear load Q takes a certain value: high average value, no amplitude; low average value, 15% amplitude; high average value, 15% amplitude; low average value, no amplitude.

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